ABSTRACT SIMPSON, JIM ANTO. A 1 Mbps Underwater Communications System using LEDs and Photodiodes with Signal Processing Capability. (Under the direction of John F. Muth.) The inability of radio frequency electromagnetic waves to propagate without attenuation in seawater has traditionally limited underwater communications to acoustics or tethered systems. High bandwidth optical communication systems have been demonstrated for terrestrial and space applications. There is growing interest to see if short range high bandwidth optical wireless systems can be made for the underwater environment. In this thesis we demonstrate a 1 Mbps optical wireless system using LEDs and PIN photodiodes that also incorporates capabilities for signal processing of the received data to be performed. Lasers and Photomultiplier tubes offer high performance, and are generally used in most underwater optical communication systems. However, these components are relatively expensive and can have large form factors. As an alternative solution the much cheaper and more compact LEDs and photodiodes are used as transmitters and receiver components. However, compared to a laser and PMT based system, such a system would be strongly disadvantaged in photon limited environments. If one assumes that photons actually reach the receiver, using signal processing techniques, optimized modulation formats, and error- correction coding, one expects that the range of the system can be extended. The development of a prototype system for the experimentation and verification of this proposition is the main motivation of this thesis. Small, compact transmitters using High Power LEDs and receivers using Si Photodiodes where the data can be digitally sampled such that signal processing techniques can be applied were constructed and demonstrated using a 12 foot, 1200 gallon tank that was also constructed for the project. It was shown that the LED and photodiode based system works well for short ranges, and that advantages can be obtained using digital signal processing. The applicability of this strategy to use digital signal processing techniques can be easily extended to higher performance Laser/PMT based systems. A 1 Mbps Underwater Communications System using LEDs and Photodiodes with Signal Processing Capability by Jim Simpson A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Master of Science Electrical Engineering Raleigh, North Carolina 2007 APPROVED BY: _________________ _________________ Dr. Leda Lunardi Dr. Brian Hughes _______________ Dr. John F. Muth (Chair of Advisory Committee) DEDICATION For: My father, Simson Yohannan, who is my biggest inspiration. ii BIOGRAPHY After attending his freshman year in Computer Engineering at UNC-Charlotte, Jim Simpson came to NC State University for the Electrical Engineering program in 2003. While at NC State, from August 2004 till August 2005, he worked on a DARPA funded project to build an underwater mine deactivating submarine along with other founding members of the NC State Underwater Robotics Club (URC). Since then he has been a part of URC, working on building autonomous underwater competition submarines for the club. From August 2004 till December 2006 he worked as an Undergraduate Research Assistant with the Bionics Group at NC State. There, he assisted in the development of a wideband wireless high density neural stimulation platform and then developed a 4-channel stimulator capable of performing different customizable stimulation strategies. Jim also developed a closed loop gastrointestinal pacemaker system in collaboration with University of Texas Medical Branch for the treatment of morbid obesity and a variety of motility disorders. Although he was able to gain valuable experience in designing high-efficiency analog and digital circuits and system level development while in the Bionics Group, his true interests in robotics and passion to develop a viable underwater communication system for the Underwater Robotics Club submarine drew him to working on a Master's thesis with Dr. John Muth. Jim graduated with a Bachelor's in Electrical Engineering in Spring 2006 from NC State University. Upon completing his Master's in Electrical Engineering, he looks forward to pursuing his Doctoral Degree in the same field. iii ACKNOWLEDGEMENTS I would like to thank: First and foremost, God, my Heavenly Father, for his infinite blessings and love Dr. Muth for his continuing support and guidance Dr. Leda Lunardi and Dr. Brian Hughes for their guidance and wisdom other students in the lab, especially William Cox and Carlo Domizioli my parents, Simson Yohannan and Rose Simson, for their love and support my friends, especially Mike and Sarah Faircloth and Rhea Motashaw iv TABLE OF CONTENTS LIST OF FIGURES ................................................................................................. vii LIST OF TABLES .....................................................................................................x CHAPTER 1. INTRODUCTION..................................................................................1 1.1. Applications ...............................................................................................................2 1.2. Component Considerations........................................................................................ 3 1.3. Hardware vs. Software Signal Processing ................................................................. 5 1.4. Proposed System Architecture................................................................................... 7 1.5. References.................................................................................................................. 8 CHAPTER 2. BACKGROUND INFORMATION............................................................9 2.1. Challenges for Free Space Underwater Optical Communications ............................ 9 2.1.1. Properties of Water................................................................................... 9 2.1.2. Absorption............................................................................................... 10 2.1.3. Scattering ................................................................................................ 14 2.1.4. Refractive Index Dependencies............................................................... 16 2.1.5. Underwater Link Budget......................................................................... 17 2.2. References................................................................................................................ 19 CHAPTER 3. TRANSMITTER AND RECEIVER FRONT-END SETUP..........................20 3.1. Transmitter Introduction.......................................................................................... 20 3.1.1. LEDs ....................................................................................................... 21 3.1.2. LED Drivers............................................................................................ 23 3.1.3. LED Secondary Optics............................................................................ 24 3.2. Receiver Introduction...............................................................................................26 3.2.1. Si PIN Photodiodes................................................................................. 27 3.2.2. Photodiode Pre-Amps ............................................................................. 29 3.2.3. Photodiode Optics................................................................................... 30 3.3. Channel Emulation Setup ........................................................................................ 32 3.4. References................................................................................................................ 33 CHAPTER 4. DATA ACQUISITION AND PC INTERFACE SETUP ..............................34 4.1. Data Acquisition Device.......................................................................................... 34 4.1.1. FPGA ...................................................................................................... 35 4.1.2. Analog to Digital Converter with Variable Gain Amplifier ................... 37 4.2. PC Interface............................................................................................................. 40 4.2.1. USB Interface.......................................................................................... 41 4.2.2. Streaming Implementation of USB.......................................................... 42 4.3. PC Software............................................................................................................. 43 4.4. References................................................................................................................ 44 CHAPTER 5. EXPERIMENTS AND RESULTS...........................................................45 5.1. Underwater optical link............................................................................................45 5.1.1. Receiver Testing...................................................................................... 46 5.1.2. Transmitter Testing................................................................................. 47 5.1.3. Receiver and Transmitter Testing........................................................... 48 v 5.1.4. Estimation of the channel attenuation using the Tx/Rx .......................... 49 5.2. LED Properties